Businessweek.com has run an interview with Star Wars creator George Lucas who states that Mark Hamill, Carrie Fisher and Harrison Ford are pretty much a lock for Disney's upcoming Episode 7 Star Wars movie:

"Asked whether members of the original Star Wars cast will appear in Episode VII and if he called them before the deal closed to keep them informed, Lucas says, “We had already signed Mark and Carrie and Harrison—or we were pretty much in final stages of negotiation. So I called them to say, ‘Look, this is what’s going on.’ ” He pauses. “Maybe I’m not supposed to say that. I think they want to announce that with some big whoop-de-do, but we were negotiating with them.” Then he adds: “I won’t say whether the negotiations were successful or not."